Trophic Interactions between Zooplankton and Sardine Mirogrex terraesanctae Populations in Lake Kinneret, Israel

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Abstract
Recent investigations in Lake Kinneret, Israel, indicated that physical, chemical and biological changes took place from 1973 onwards. The modifications observed in zoo-plankton populations were analyzed and related to concomitant changes noted in the population of M. terraesanctae (Kinneret sardine), which is the main predator of zooplankton. The cladocerans Diaphanosoma sp., Bosmina spp. and Ceriodaphnia spp. were preferential food sources for Mirogrex when compared with Copepoda. In 1973 and in the 1st half of 1974, the biomass and productivity of Cladocera were higher (30-170%) than the average for 1969-1972. This was accompanied by a continual increase of purse-seine catch-per-unit-effort of sardine up to to 1454 kg/boat-day during 1972-1976. Small and young Mirogrex fishes were more frequent in the 1974-1975 catches than in previous seasons. The increase of the Mirogrex population at the expense of the Cladocera caused a 26% decrease of productivity of the latter organisms compared to the interannual average (1969-1975). It also affected the age structure of the Copepoda population: the adult females became less abundant but they showed a higher fecundity.

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